Embodiments of the invention relate generally to a Micro-Electro-Mechanical Systems (MEMS) device and, more particularly, to a metal MEMS device where the mechanical element is comprised of an electroplated metal that is deposited onto an seed layer consisting of a refractory metal. In one embodiment, the seed layer is left intact on a bottom surface of the mechanical element to act as an electrical contact for the mechanical element in an embodiment where the MEMS device is constructed as a MEMS switch.
MEMS is a technology that in its most general form can be defined as miniaturized mechanical and electro-mechanical elements (i.e., devices and structures) that are made using the techniques of microfabrication. The critical physical dimensions of MEMS devices can vary from well below one micron on the lower end of the dimensional spectrum, all the way to several millimeters. Likewise, the types of MEMS devices can vary from relatively simple structures having no moving elements, to extremely complex electromechanical systems with multiple moving elements under the control of integrated microelectronics, with free-standing MEMS structures or “beams” often acting as relays, for example.
With respect to MEMS devices having moving elements, such a moving element may be in the form of a free-standing and suspended. MEMS structure that is configured as a cantilever with a first end anchored to a substrate (e.g., fused silica, glass, silicon substrates) and a second free end having a contact. When the MEMS device is activated, the free-standing MEMS structure moves its contact against a substrate contact on the device substrate and under the MEMS structure contact.
In fabricating/forming such a metal MEMS free-standing structure, an electroplating process is employed whereby a metallic material is electroplated onto an electrically conductive metal layer, i.e, a “seed layer,” which is necessary to carry the current which facilitates the metal deposition at the wafer surface. Typically, this metal seed layer must be removed at the end of the fabrication process by one of several metal etching methodologies (i.e. wet chemical etching, reactive ion etching). Failure to fully remove this seed layer can result in device failures such as shorting between electrical elements, and it is recognized that the method for etching the seed must be designed such that it does not damage the MEMS structures present on the substrate.
With specific regard to MEMS switches, it is further recognized that—in operation—the contacting of the free-standing structure with the substrate contact can cause the free-standing structure (i.e., a contact of the free-standing structure) to experience mechanical wear due to repeated physical impact with the substrate contact, heating of the free-standing structure contact by joule heating, and electrical discharges between the free-standing structure contact and the substrate contact. This wearing of the free-standing structure contact can eventually lead to reliability issues in the MEMS switch.
Therefore, it is desirable to provide a free-standing MEMS structure that may be fabricated by electroplating the free-standing MEMS structure onto a seed layer, with the seed layer also acting as a contact material, thus eliminating the need to remove the seed metal at the end of the process. It is also desirable that this seed layer be tailored so that it acts as the contact layer while at the same time it does not contribute significantly to the mechanical and electrical properties of the beam/seed layer structure over a range of temperatures. For example, the stress in the films affects the planarity of the released MEMS structure, and so, must be controlled. It is still further desirable that the seed layer, in acting as an ohmic contact for the free-standing structure, be formed of a refractory metal or refractory metal alloy exhibiting properties such as melting temperature above 1850° C. and a melting voltage above 0.4 V, such that it exhibits increased resistance to mechanical wear and exhibits increased longevity when exposed to at high temperatures and electrical discharges.